1. Field of the Invention
This invention relates generally to inertial measurement units. More specifically, this invention relates to accelerometers and gyroscopes.
While the present invention is described herein with reference to a particular embodiment for particular applications, it is to be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of the present invention, will recognize additional embodiments and applications within the scope thereof.
2. Description of the Related Art
Inertial navigation has long been recognized as an advantageous method of navigation inasmuch as no external reference is required. Navigation is accomplished by sensing the motion of the vehicle and calculating the change of position after initial alignment. This requires accurate instrumentation for detecting rotational motion (angular turning rates) and linear or translational motion relative to a stabilized frame of reference. Gyroscopes (gyros) are typically used to detect angular turning rates. The gyro is typically implemented in a servo-loop to stabilize a frame of reference on which accelerometers may be mounted to measure linear motion. The stabilized frame of reference may be a platform on or within a vehicle or the vehicle itself.
In its simplest form, a gyro may be brought of as a rapidly spinning rotor or flywheel supported on a mount which allows freedom of tilt of the spin axis relative to the base. The rapid spinning of the flywheel gives the spin axis of the gyro a stubborn resistance to angular deflections. The gyro thus tends to preserve its original orientation which with proper alignment is also the desired vehicle or platform frame of reference.
A single degree of freedom gyro is one having a spin axis that is allowed to move in one direction and restrained in the others. A two axis gyro is one having an axis of rotation capable of deflection in either of two planes or axes. Similarly, a three axis gyro is one having an axis of rotation capable of deflection in either of three axes. Such gyros are therefore said to have two and three degrees freedom resectively and are known as free gyros.
Since three axes of stabilization are required to stabilize a frame of reference, either three single axis gyros, two two axis gyros or one three axis gyros is required. The use of three single axis gyros was initially prefered to multiple axis gyros until techniques were developed to model and eliminate cross-coupling between axes in multiple axis gyros. U.S. Pat. No. 3,823,990 issued to P. J. Gilinson Jr. on July 16, 1974 discloses a three axis gyro (see FIG. 13). Gilinson typifies conventional gyros in that it operates on the principle of a spinning flywheel. In some applications such as space however, the weight associated with the gyro flywheel is particularly undesireable. Thus there is a need to reduce the weight and cost associated with conventional gysoscopes.
As mentioned above, the accelerometer is the second typically essential component of conventional inertial measurement units. An accelerometer is a device that measures the acceleration of the vehicle in one direction relative to the stabilized frame or reference. Conventional mechanical accelerometers include a spring or hinge adapted for deflection in response to acceleration. A mechanical, optical, inductive or capacitive pickoff is used to detect motion of the hinge. To meet manufacturing tolerances, the hinge or spring is biased. Such factors as vibration, shock, temperature variations, and time dependent stress relief shift the bias and introduce errors in the output calculation. Conventional mechanical accelerometers require frequent calibration and alignment relative to the gyros. In addition, these accelerometers are difficult to manufacture as the spring or hinge must typically be machined, ground, or polished to exact specifications. The difficulty in doing so, lowers the yield and raises the cost of producing such instruments.
A further disadvantage of conventional mechanical accelerometers results from the need to physically mount the accelerometer on the frame of reference, typically either a stabilized platform or the vehicle itself. To protect the instrument from vibration, rubber isolators are typically used. Over time, the isolators often lose resiliency and settle in a manner that causes misalignment of the accelerometer. This provides an additional source of error.
The degree of damping also imposes a limitation on gain. A high gain system requires a tight servo. That is, since a high gain system is quick to respond to vibration, inadequate damping may lead to a degradation in performance and/or undesired oscillation. Thus, it would be desireable to provide a system with high damping thereby permitting high servo gain with the associated improvement in system performance.
Conventional mechanical accelerometers are also subject to resonance from the spring constant associated with the accelerometer or the restraint. Resonance is undesirable as an additional source of output error.
It is desireable therefore to provide a single, suspended, integrated, accurate, inexpensive, lightweight instrument capable of providing a three axis angular turning rate detector (or reference frame stabilizer) and a self-damping nonmechanical three axis accelerometer with no mechanical supports or restraints.